Most people on this board don't use the internal mics much, so we may not have the best experience there. If you are interviewing people for news, that's different from what many of us are doing (loud concerts).
<pure_speculation>
I kind of doubt the PMD661 has great internal mics, or that anyone here has used them much. I suspect they are in designed for "put this thing in the middle of the conference room table to record this business meeting", and not really designed for music. I suspect the R26 are decent. Not sure about the DR-100. I expect the Sony D50 mics are decent.
</pure_speculation>
Also consider the lower end Zoom H4 or H4n. The internal mics on those are pretty good... because I know people who have used those internal mics, and I've been impressed with the results.